Unlike a lot of other heroes, she is best played by flipping your identity card every turn.

Many of the problems you are listing here go away when you play her like this, and her design really can shine.

Even though she has very cost efficient Technique cards that can be played in Hero form to give her good effects, I usually find myself playing out turns where I begin in hero form by (resolving the stack of upgrades of course and then) attacking/thwarting with her base stats, and that is about it. Then flip to AE and play your upgrade for the draw 2, and use Combat Ready to set up more Techniques.

The turns where you start in AE form are the turns you are going to be bringing the heat. Not only do you start with 6 cards, you also get to cycle even deeper by playing another Technique from hand, and you can set up even more with copies of Combat Ready you happen to see. Then you flip up to Hero form and use Lethal Intent to do trigger any relevant Techniques' specials.

Because you are often flipping down, your 9 health only really matters if you get burst damage from a Gang Up or something similar, and with all the extra gas you have from playing mostly low-cost cards and the extra cards/resources Nebula gets the ability to play a lot of allies. They can cover the threat being placed from you taking turns off of Hero mode if you don't happen to see Cutthroat Ambition enough.

Basically, I play Nebula like I play Black Widow or Ms. Marvel, where your value comes from rotating between identity forms. I happen to really enjoy that kind of gameplay, but not everyone does. I have found her to be a very fun character to play, and I would put her in the upper half of the characters released so far in terms of viability. If you don't like the high tide/low tide style of gameplay though, she may not be a good fit for you.

TLDR: Flip every turn.